Radiation-emitting devices are generally known and used for radiation therapy in the treatment of patients, for example. Typically, a radiation therapy device includes a gantry which can be swiveled around a horizontal axis of rotation in the course of a therapeutic treatment. A linear accelerator is located in the gantry for generating a high-energy radiation beam for therapy. This high radiation beam can be an electron radiation or photon (X-ray) beam. During treatment, the radiation beam is provided on one zone of a patient lying in the isometer of gantry rotation.
The goal of radiation treatment planning is to maximize the dose to the target volume while protecting radiation-sensitive healthy tissue. The X-ray beam intensity often varies over the treatment field by placing an X-ray absorber in the beam's path. This allows the target volume to be placed in regions of high beam intensity, while the surrounding radiation-sensitive tissue is protected by placement in low intensity regions. A simple example is a wedge-shaped isodose distribution, which has been found to be clinically useful in treatment plans.
One frequently used method is to place a physical wedge accessory (i.e., a wedge-shaped absorber) in the X-ray beam path that exponentially decreases the beam intensity laterally across the treatment field. A desirable wedgeshaped isodose distribution results. The "toe" of the wedge (i.e., where the thickness of the wedge is the smallest) produces the high beam intensity region, since this portion of the beam has the least attenuation.
The use of the physical wedge accessory has some negative side effects, however. The primary beam intensity is reduced at the target volume; thus, treatment times are increased. Further, scattering of the beam outside the treatment field causes additional dose to be delivered outside the target volume. It also introduces a spatial energy dependence (i.e., hardness) to the beam, affecting the depth at which the radiation is absorbed across the treatment field. Additional time and effort are required to design, validate, manufacture, install remove, and store the accessories. In addition, only a limited number of wedge angles are available.
The virtual wedge function integrated into some treatment devices, such as MEVATRON and PRIMUS systems from Siemens Medical Systems-Oncology Care Systems, Concord, Calif., is used to achieve an accumulated dose profile and isodose distribution similar to that of a physical wedge accessory. The virtual wedge function is accomplished by controlling the travel of a secondary collimator jaw and the X-ray beam intensity during irradiation. The virtual wedge scheme eliminates most of the problems associated with the physical wedge. However, some problems still exist in getting a uniform dose at a desired depth.
An example of wedge isodose distributions is shown in FIG. 1. Wedge angles for wedge isodose distributions are presently defined at a 10 cm (centimeter) depth based on where the isodose crosses the central axis. A line drawn tangent to the isodose defines the nominal wedge angle (.alpha.). Virtual wedge treatments regularly use the 10 cm depth in defining the wedge angle. But, treatment areas are often at different depths, such as 5 cm, which affects the dose distribution, but which is not readily accommodated in the virtual wedge treatment to ensure uniform dose delivery.
Accordingly, what is needed is a method and system for optimizing wedge angles in a virtual wedge treatment.